Contemporary shoes provide a complex combination of cushion, traction, and body-fitting elements that facilitate the natural progression of the foot along the ground during wear. When walking or running, the first part of the foot to strike the ground is the heel at the instant of “heelstrike.” Then the foot rolls forward from the heel region, through the midfoot region and the ball of the foot, and finally to the toe region where the foot breaks contact with the ground in a “toe-off” action. In addition to the above-described progression from the heel to the toe, the foot typically rolls from the outside to the inside (lateral side to the medial side), a process called “pronation” which disperses some of the energy generated during the impact of the foot with the ground. In some cases, the foot may not pronate enough, a condition called “underpronation” or “supination.” Furthermore, persons with underdeveloped arches may suffer from “overpronation,” wherein the foot rolls inward excessively. Either condition is unhealthy for the foot and can cause shin or joint pain.
A shoe generally includes a “sole assembly” that provides the main weight bearing support at the plantar region of the foot, and an “upper” that connects to the sole assembly and surrounds other areas of the foot. The sole assembly typically includes an outsole and a midsole. The outsole is generally the portion of the sole assembly that makes contact with the ground, while the midsole is positioned just above (as the shoe normally touches the ground) the outsole and usually provides a cushioning affect. An insole, typically separate from the sole assembly, is normally positioned above the midsole and within the upper of the shoe to make contact with the wearer's foot. As the main support for the wearer's weight, the sole assembly of a shoe plays an important role in providing a healthful, natural stride.
Early sole assemblies included a continuous midsole formed of a single piece of foam cushion material that formed a continuous bottom surface on which a tread outsole was provided. This design was initially adopted by the footwear industry because of its simple structure, which made manufacturing easy and cost effective. However, conventional continuous midsole designs required thick slabs of foam material in order to effectively absorb and disperse impact and propulsion forces generated during athletic use. Further, these conventional unitary sole designs were heavy and held the foot relatively high above the ground surface, thereby reducing lateral stability. Based on these characteristics, the footwear industries has perceived the unitary sole to be unsuitable for athletic footwear and has developed alternative designs for performance footwear.
For example, a split sole assembly design has evolved as an industry standard for lightweight athletic shoes. With split soles, the heel and toe sections of the midsole are separated by a pronounced arch or deep groove, and a lightweight rigid shank is typically used to structurally connect the heel and toe regions of the midsole. As the shank allows removal of a substantial portion of the midsole foam, the split sole design generally provides a lightweight sole assembly. However, the present inventors have recognized that the shank region of the split sole is typically raised from ground contact (particularly along a periphery), which reduces traction capabilities and lateral support in this region of the split sole design. Further, the arched shank region requires a vertical dimension that increases the overall height of the split sole assembly, thus further reducing lateral stability. Yet another problem with the split sole is that the discrete heel region of this design generally concentrates heel strike forces in a small area making it more difficult for the wearer to naturally transition heel impact forces to midstance and toe off during stride.
Various known footwear devices have also been developed to enhance performance of athletic shoes. For example, U.S. Pat. No. 4,821,430 to Flemming et al. describes a heel counter having a U-shaped side wall extending about a heel portion of a shoe upper, and a flexible membrane connected to a bottom portion of the U-shaped wall. Under the wearer's weight, the membrane flexes to draw the U-shaped wall inward to laterally support the wearer's foot. However, while not specified in the '430 patent, such support features have been implemented only in non-unitary sole designs such as the split sole described above. Moreover, the heel counter does nothing to reduce the overall height of the sole assembly, and the thin membrane provides only weak support for wearer's heel and little dispersion of heel impact forces. Still further, the flexible membrane may be a barrier to ventilation of the foot.
Footwear features have also been developed to provide improved ventilation to the wearer's foot. For example, air passages that extend from a bottom surface of the sole assembly to an interior of the shoe upper have been used to increase air flow to the wearer's foot. As these through holes remove outsole and midsole material they also reduce the weight of the sole assembly. Again, however, these ventilation features have been implemented in non-unitary sole designs. Moreover, passages that extend from the bottom of the outsole can function as suction cups on the outsole, thus causing additional resistance to lifting the foot, especially in wet or muddy areas. While side surface air passages are also known, these passages typically extend only from a lateral to medial side of the sole assembly, thus providing no ventilation to the interior of the shoe. Further, placement of side passages is typically based only on weight considerations or aesthetics, making other footwear design considerations necessary to address unique characteristics of a wearer such as under pronation or over pronation.